1. Field of the Invention
The present invention relates to a spectrum spread communication system for used in a digital automobile telephone system, a digital cellular telephone system, personal communication services (PCS) and the like.
2. Description of the Related Art
A spectrum spread communication system is a transmission system in which information is transmitted to be spread into a sufficiently wide band in comparison with minimum necessary band width. Further, this system is a communication system having excellent conversational secrecy, official secrecy and non-interference. A direct spread (DS) system is a spectrum spread communication system in which information is directly multiplied by a spread code.
In a cellular radio communication system such as an automobile telephone, a cellular telephone and the like, an FDMA (Frequency Division Multiple Access) system, a TDMA (Time Division Multiple Access) system and the like are known as multi-dimensional access technology when a plurality of stations simultaneously communicate with one another in the same frequency zone. A CDMA (Code Division Multiple Access) system using the spectrum spread communication system is advantageous in that high efficiency of frequency utilization can be obtained in comparison with any other technology and that more users are accommodatable.
In the CDMA system, it is effective to use codes having higher orthogonality as spread codes to increase capacity. However, Walsh codes and orthogonal Gold codes are known as those having high orthogonality but the number of them is limited to the same number of code lengths. Therefore, in order to secure the number of spread codes to be allotted to users, U.S. Pat. No. 5,103,459 discloses to combine short codes, the period of which is equal to the symbol length of information, and long codes, the period of which is greater than the former to be used. In this case, a long code used at one base station is one code in the forward link, and different long codes are allocated to respective base stations. Accordingly, the orthogonality of the whole user within the same cell can be maintained, and since a signal in any other cell is spread with a different long code, the signal is become noise to thereby suppress interference. In such a system using the long code, the mobile station is required to acquire and hold long code synchronization to keep in communication.
In the cellular system, when the power supply of the mobile station is turned on, when the communication between the mobile station and the base station is broken, or when the mobile station conducts a handover to specify a base station to be switched for changing the base station to be communicated from one to another accompanying with its position change during communication, the mobile station has to specify which base station is closest at present and is under best condition to communicate.
In the cellular system using the CDMA system, U.S. Patent discloses a conventional method to conduct long code synchronization and a call range decision, in which all of the base stations offset the pilot channel spread with the same long code and transmit the offset pilot channel so as not to be consistent with the phase (timing) of a spread code, and the mobile station correlatively detects the whole phase of spread code of the received pilot channel so as to specify a base station with the phase at the highest correlation value as the closest one. However, it is required in this system that synchronization has been obtained between the base stations, and it is necessary to distribute timing as a reference at the whole base stations. Moreover, the phase of a selectable long code is restricted and when the number of base stations is increased, the offset of a base station to be newly installed has to be selected so as to prevent overlapping in consideration of the long code offset value of the surrounding base stations and installation design becomes necessitated. Accordingly, this system is not fit for an inter-cell asynchronous system wherein installation design is unnecessary and wherein installation of new base stations can be dealt with flexibly in view of an increase in traffic and so forth.
Since the aforementioned conventional system is intended to acquire initial synchronization using the pilot channel superposed on any other user channel, more integration time is required when the correlation of the pilot channel is detected in order to not only suppress the interference component derived from other users but also raise the reliability of the correlation value. Consequently, time required for the initial synchronization is increased. Although it may be attempted to shorten the time required for the initial synchronization by increasing the power of the pilot channel, the pilot channel tends to interfere with other channels often and thus communication quality is deteriorated.